1. Field of the Invention
The present invention presents a robot calibration apparatus and method for calibrating a robot arm, and more particularly, a robot calibration apparatus and method for calibrating a robot arm by determining moving errors.
2. Description of the Prior Art
With the rapid development of industrial production technology, factory production lines are having robot arms perform product handling, arrangement, or assembly in place of human laborers to increase speed of production, stabilize product quality, and reduce labor cost. The robot arms further includes the use of visual system to determine a position and orientation of an object. The visual system guides the robot arm to automatically and accurately grab an object in the production process. Therefore, a method to reduce positioning error of the visual system and a method to calibrate the accuracy of the movement of the robot arm have become an important topic.
FIG. 1 illustrates a diagram of a calibrating plate 10 for a robot arm according to a prior art. The calibrating plate has a chessboard having interchanging black squares 11 and white squares 12. During calibration of the robot arm using the calibrating plate 10, a visual system of the robot arm is used. The visual system comprises an eye in hand camera configured to guide the robot arm when grabbing an object and an eye to hand camera configured to monitor a working environment of the robot arm. When calibrating using the eye in hand camera fixed on the robot arm, the eye in hand camera is brought close to the standard calibration plate 10 by moving the robot arm to capture an image of the standard calibration plate 10. According to the captured image, the position of the robot arm relative to the standard calibration plate is calculated to complete the calibration of the robot arm.
There are several calibration methods using the standard calibration plate. For example, U.S. Pat. No. 6,985,175 is configured to use two cameras to capture images of the status of the standard calibration plate and perform comparison to calibrate the two cameras. Another example, CN102927908 is configured to use a laser apparatus to project light strip for performing alignment of standard calibration plate. The camera fixed on the robot arm is used to capture image of the light strip. The laser apparatus is moved to another position and another image of the light strip projected by the laser apparatus at another angular position is captured. The intersection of the two light strips from two different angular positions is used to calibrate position of the robot in a three-dimensional space.
However, the standard calibration plate of the prior art has a fixed number of black and white squares in the chessboard pattern. If the robot arm is set on a limited space and the distance between the robot arm and the standard calibration plate is fixed, the visual system arranged on the robot arm is not able to capture the entire image of the standard calibration plate. It would not be possible to determine the orientation and position of the visual system using the partial image of the standard calibration plate. Thus, it is hard to determine the correct position of the visual system relative to the standard calibration plate to perform calibration. Therefore, existing method of calibrating robot arm still has problems that need to be solved.